The present invention relates to an optical quality control system for controlling the quality of a high refractive index translucent or transparent solid. The invention may in particular be used for controlling the quality of gemstones, sapphires, sapphire ingots, technical optical materials and crystals.
An apparatus and method for inspecting a precious stone, which is at least partially translucent, is known from U.S. Pat. No. 4,049,350. The stone is immersed in a liquid having the same or a similar refractive index. Thereafter it is scanned with a focused laser beam emitted from a laser source, and a photosensitive device comprising photoconductive cells is mounted within a plate, which is configured to carry the stone. The photosensitive device is arranged opposite of and in-line with the laser beam. Since the laser beam only allows a punctual control of the precious stone it is suggested to move the laser source and the precious stone relative to one another during the controlling operation. For this reason a motor configured to rotate the plate so that the precious stone may be rotated and a mechanism to move the laser source are provided, thus creating a spiral scanning trajectory. The laser beam is preferably moved across the stone via a pivotable mirror. Since it is preferred that the precious stone is fully immersed in a liquid having a similar refractive index as the stone, the angle of refraction of the light beam is low when it enters the precious stone and thus it passes through the stone without losing much intensity. On the other hand when the laser beam hits a bubble or inclusion it is dispersed and the intensity of the laser light passing straight through the stone will be reduced. This reduction is measured by the photosensitive device.
The high refractive index liquid reduces the reflection from the outer surfaces of the stone and the refractive bending and separation of the light when it enters the precious stone.
The device described in U.S. Pat. No. 4,049,350 does not allow determining the exact spatial position of the defect with one scan, since only the x-, and y-coordinates (horizontal) of the defect can be determined and not the z-coordinate (vertical). It is thus suggested to scan the precious stone a second time in a different direction. A second scan also allows determining whether or not a possible detected defect has its origin in a particle in the liquid or on the surface of the precious stone.
The above described process is slow and costly in view of the three dimensional point scanning process. Further, due to the nature of the method comprising a spiral trajectory of the laser beam, certain areas within the precious stone may be missed when a scan is made. Moreover, the photosensitive device is exposed and arranged within the liquid, which is potentially chemically aggressive. In case there is a problem with the photosensitive device the reparation may be costly and complex.